The biological springs that act in parallel and in series with skeletal muscle contractile elements can significantly influence the force, power and displacement of muscle fibers during locomotion. All movements involve dynamic interaction between passive elastic structures and muscle contractile elements, but our understanding of the consequences of this interaction for normal gait is very limited. Our long-term goal is to define rules for muscle-tendon function that govern force production during movement. The work proposed here focuses on the role of passive elastic structures in locomotor activities that require mechanical energy absorption (e.g., deceleration, downhill running, jump landing). We hypothesize that energy storage and recovery in passive elastic structures buffers muscle energy absorption to limit the peak powers, forces and lengthening velocities experienced by muscle contractile elements. This buffering function may protect against damage from the high forces associated with active muscle lengthening. A unique animal model system provides direct measurement of force, length and power in individual muscles during locomotion. This system allows us to distinguish length changes in tendons from those of muscle contractile elements. The first hypothesis to be tested is that tendons redistribute in time the work done on muscle contractile elements, thereby limiting peak muscle powers and forces during lengthening (Specific Aim 1). The possible role of parallel elastic elements in energy absorbing movements will also be investigated. We will combine in situ determinations of the passive muscle length- tension relations with in vivo measurements of muscle length and force to determine whether muscle parallel elastic elements can contribute force during locomotor movements that involve muscle lengthening (Specific Aim 2). Finally we will determine whether the energy-buffering function of elastic elements is adaptable, by characterizing the response of muscle and tendon to a downhill and uphill running training protocol (Specific Aim 3).Changes in the passive properties of muscles and tendons occur in association with muscle spasticity secondary to stroke and spinal cord injury. A fundamental understanding of the mechanical function of passive elastic elements during healthy gait will inform the design of rehabilitative strategies to improve muscle- tendon function. The design of prosthetic devices will also be served by a knowledge of the mechanical behavior of healthy muscle-tendon units. PUBLIC HEALTH RELEVANCE. Changes in the passive properties of muscles and tendons occur with aging, and in association with muscle spasticity secondary to stroke and spinal cord injury. A fundamental understanding of the mechanical function of passive elastic elements during healthy gait will inform the design of rehabilitative strategies to improve muscle- tendon function. The design of prosthetic devices will also be served by a knowledge of the mechanical behavior of healthy muscle-tendon units.